Conventionally, there have been proposed various types of tire-acting-force detection apparatuses for detecting force acting on a tire of a wheel unit in a vehicle such as an automobile. For example, Japanese Patent Application Laid-Open (kokai) Nos. 2003-14563, 2005-249517, and 2005-274492 disclose tire-acting-force detection apparatuses previously proposed by the applicant of the present application. Each of the disclosed tire-acting-force detection apparatuses is interposed between a wheel which holds a tire at a peripheral portion thereof and a holing member which holds the wheel such that the wheel can rotate about a rotation axis, and is configured to indirectly detect forces acting between the wheel and the holding member, to thereby detect force acting on the tire.
The proposed tire-acting-force detection apparatus can detect force acting on the tire on the basis of force acting between the wheel and the holding member. However, in a tire-acting-force detection apparatus of such a type, a wheel-side member fixedly connected to the wheel and a holding-member-side member fixedly connected to the holding member are indispensable. Further, the tire-acting-force detection apparatus of such a type requires a member which allows the wheel-side member and the holding-member-side member to move relative to each other when a force acts between the wheel and the holding member due to a force acting on the tire, or a member which transmits stress between the wheel-side member and the holding-member-side member. Moreover, a detector must detect the relative displacement between the wheel-side member and the holding-member-side member or the stress transmitted therebetween. Accordingly, the above-described conventional tire-acting-force detection apparatuses have problems of complicated structure and high cost.
In general, in a vehicle such as an automobile, in order to allow maintenance work such as replacement of the tire, the wheel is fixedly connected to the holding member by means of (generally, four) connection means each including a connection element such as a bolt. Accordingly, when a force from a road surface acts on the wheel via the tire, the greater portion of the force is transmitted to the holding member via the connection means. A stress transmitted to the holding member via the connection means corresponds to the force acting on the tire. Accordingly, the force acting on the tire can be estimated from the stress acting on each connection means without interposing a complicated tire-acting-force detection apparatus between the wheel and the holding member.